Optical fibre management system

ABSTRACT

An optical fibre management system for manages a plurality of optical fibres. The system is such that the fibre(s) of each single circuit is/are routed separately from the fibres of other circuits, whereby optical signals carried by any given single circuit are not degraded by handling operations carried out on the fibres of other circuits.

RELATED APPLICATIONS

This is a continuation application of U.S. patent application Ser. No.08/278,217 filed Jul. 21, 1994 entitled "OPTICAL FIBRE MANAGEMENTSYSTEM" now U.S. Pat. No. 5,588,076 which is a continuation-in-part ofU.S. patent application Ser. No. 08/202,190 filed Feb. 24, 1994 entitled"OPTICAL FIBRE MANAGEMENT SYSTEM" now U.S. Pat. No. 5,548,678 and namingMessrs. Frost and Kerry as inventors.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to an optical fibre management system, and inparticular to an optical fibre splitter array sub-assembly forincorporation in the node of an optical fibre telecommunicationsnetwork.

2. Related Art

In the United Kingdom, the telecommunications network includes a trunknetwork which is substantially completely constituted by optical fibre,and a local access network which is substantially completely constitutedby copper pairs. Flexibility in the copper access network is provided attwo points en route to the customer; firstly, at street-side cabinetsserving up to 600 lines; and secondly, at distribution points servingaround 10-15 lines. In total, the network has about 250,000 km ofunderground ducts, 83,000 cabinets, 3.1 million distribution points and3.7 million manholes and joint boxes. Eventually, it is expected thatthe entire network, including the access network, will be constituted byfibre.

The ultimate goal is a fixed, resilient, transparent telecommunicationsinfrastructure for the optical access network, with capacity for allforeseeable service requirements. One way of achieving this would be tocreate a fully-managed fibre network in the form of a thin, widespreadoverlay for the whole access topography as this would exploit theexisting valuable access network infrastructure. Such a network could beequipped as needs arise, and thereby could result in capital expendituresavings, since the major part of the investment will be the provision ofterminal equipment on a `just in time` basis. It should also enable therapid provision of extra lines to new or existing customers, andflexible provision or reconfiguration of telephony services.

In order to be completely future proof, the network should be singlemode optical fibre, with no bandwidth limiting active electronics withinthe infrastructure. Consequently, only passive optical networks (PONs)which can offer this total transparency and complete freedom forupgrade, should be considered.

The most common passive optical network is the simplex single star, withpoint-to-point fibre for each transmit and receive path, from theexchange head end (HE) to the customer network terminating equipment(NTE). This network design has been used throughout the world and meetsall the access criteria. It involves high fibre count cables, and uniqueelectro-optic provision at HE and NTE for each customer. The resultinginherent cost can only be justified for large business users, whogenerally also require the security of diverse routing, which increasesthe cost still further.

The advent of optical splitters and wavelength-flattened devices hasenabled the concept of the PON to be taken one step further. Thesepassive components allow the power transmitted from a single transmitterto be distributed amongst several customers, thereby reducing andsharing the capital investment. In 1987, BT demonstrated splittertechnology in a system for telephony on a passive optical network(TPON), with a 128 way split and using time division multiplex (TDM)running at 20 Mb/s. This combination enabled basic rate integratedservice digital network (ISDN) to be provided to all customers. Inpractice, the competitive cost constraint of the existing copper networkprecludes domestic customers from having just telephony over fibre, dueto the high capital cost of equipment. This may change in the future. Inthe meantime, telephony for small business users (for example thosehaving more than 5 lines) can probably break this barrier.

The wider range of services and higher capacity required by businesscustomers makes a 32-way split more attractive for a 20 Mb/s system andthis has been demonstrated by BT's local loop optical field trial(LLOFT) at Bishop's Stortford.

In summary, the use of splitter based PON architecture will reduce thecost of fibre deployment in the access network. When compared withpoint-to-point fibre, savings will result from:

(i) reducing the number of fibres at the exchange and in the network;

(ii) reducing the amount of terminal equipment at the exchange;

(iii) sharing the cost of equipment amongst a number of customers;

(iv) providing a thin, widespread, low cost, fibre infrastructure; and

(v) providing a high degree of flexibility, and allowing `just in-time`equipment and service provision.

Additionally, PON architecture can be tailored to suit the existinginfrastructure resources (duct and other civil works).

Total network transparency will retain the option for future services tobe provided on different wavelengths to the telecommunications, whichfor TPON is in the 1300 nm window. By transmitting at other wavelengths,other services, such as broadband access for cable television and highdefinition television, or business services, such as high bit rate data,video telephony or video conferencing, can be provided. The hugebandwidth potential of fibre promises virtually unlimited capacity forthe transparent network. Eventually, it will be possible to transmithundreds of wavelengths simultaneously, as the development of technologyin optical components, such as narrow band lasers, wavelength divisionmultiplexers (WDMs), optical filters, fibre amplifiers and tunabledevices, moves forward.

For this potential to remain available, and for the access network to beused to provide many and varied services, then it must be designed andengineered to provide very high levels of security and resilience. Evenfor simple POTS, advance warning and live maintenance are essential tolimit disruption.

Resilience implies separacy of routing, and exploiting the existinginfrastructure of underground ducts and other civil works is a primerequirement of the design philosophy. Analysis of this resource hasindicated that separacy, from creating primary ring topographies, couldbe achieved by linking the spine cables which currently feed manyprimary connection points (PCPs) in the existing star type network.

In order to create rings from the existing star configurations, somelocalities will have existing ducts that will allow the link cables tobe installed. In BT's suburban networks, analysis has shown that onaverage 60% of PCPs can be served on rings using existing ducts; and, byadding new ducts links of 200 m or less, a further 30% can be covered.In some cases, there will be natural or man made boundaries wherephysical rings cannot be provided, and in these cases duplicate fibresin the same duct route, i.e. across rivers or over railway bridges, maybe the only choice.

The architecture adopted for the PON topography will be influenced bytransmission techniques, and the availability of suitable splittercomponents. Transmission options are simplex (two fibre paths), duplex,half duplex or diplex (single fibre path).

Simplex working increases the complexity of the infrastructure due tothe two fibres per circuit required. However, it benefits from thelowest optical insertion loss, due to the absence of duplexing couplers;and the lowest return loss, since such systems are insensitive toreflections of less than 25 dBm with separate transmit and receivepaths. Duplex and half duplex working each have an insertion losspenalty of 7 dB from the duplexing couplers, and diplex working replacesthese with WDMs, with a reduced penalty of 2 dB.

In view of the long term aim to provide a total fibre infrastructure,and the present early state of passive technology components, it isconsidered beneficial to opt for simplex working and a relatively lowlevel of split (≦32) for PON networks.

In an optical fibre communications system, transient changes in opticalattenuation can cause transmission errors. These changes are caused bytransient bend loss at various points along the fibres of the system,and the extent to which traffic along a given fibre is disturbed isdependent on such physical variables as the total loss incurred and theduration of the transient. Transient losses occur mainly because offibre handling and maintenance procedures, particularly in the regionsof fibre splices. Thus, when multi-fibre splice trays are opened, and/orfibre is handled, attenuations of up to 10 dB can be observed. Forexample, a typical splice tray used in optical communications systemscontains 24 splices, and handling any one of the splices for maintenancepurposes causes transient losses in adjacent fibres. This problem isillustrated in FIG. 15 of the accompanying drawings, which plots theprobability of the occurrence of error against the system margin, atboth 1550 nm and 1300 nm for a procedure which includes opening atypical 24 fibre splice tray, and running a finger along the splices.Error loss measurements are made of the fibres at splice position 14, asthis splice position is almost in the centre, and so is more susceptibleto transient losses than other splice positions. As shown in FIG. 15, atthe optimum operating position of the receiver making the errormeasurement (that is to say at a system margin of 0 dB) at both 1550 nmand 1300 nm there is a large percentage error occurrence as a result ofthe transient losses caused by the fibre handling. As the system marginis increased, the percentage error occurrence falls at both 1550 nm and1300 nm, but there is still a significant percentage error occurrence at1550 nm even as the system margin approaches the dynamic range(typically 15 dB) of the receiver. The normal operating position of thereceiver is the optical power nominally detected at the receiver toachieve a bit error rate (BER) of 10⁻⁹ or better. The results at 1550 nmare far worse than those at 1300 nm, due to increased bend sensitivityat 1550 nm and hence larger transients. This is potentially worrying ifsplice trays are to be installed in a system operating at 1300 nm withlater provision for operation at 1550 nm. This is because there may be apoint at which a system may operate with no handling errors at 1300 nm,but will show a serious handling error performance at 1550 nm due to theincreased bend loss sensitivity of fibres at 1550 nm. This would lead toa need for an increased system margin at 1550 nm to compensate for thegreater losses at 1550 nm. This is undesirable because it results inlower power budgets due to a higher required incident optical power.

SUMMARY OF THE INVENTION

The aim of the invention is to provide a fibre management system whichdoes not suffer from the type of transient losses referred to above.This is accomplished by what may be termed "single circuit management"In the context of an optical fibre communications system, a singlecircuit is one or more fibres carrying optical signals between twodifferent locations. Thus, a single circuit may be constituted by asingle fibre which connects a transmitter/receiver pair at a firstlocation and a transmitter/receiver pair at a second location.Similarly, a single circuit may be constituted by a first fibreconnecting a transmitter at a first location to a receiver at a secondlocation, and a second fibre connecting a transmitter at the secondlocation to a receiver at the first location. Again, a single circuitmay be constituted by a plurality of optical fibres interconnecting atransmitter at a first location and a receiver at a second location, anda plurality of fibres connecting a transmitter at the second locationand a receiver at the first location. Single circuit management can alsobe thought of as starting at the point where a circuit breaks out fromother circuits, and continues until a multiple circuit is reformed. Inother words, a single circuit is one or more fibers carrying opticalsignals between a first separation point to either a customer terminalor a second separation point.

The optical fibre communications system of the invention provides singlecircuit management and so ensures that optical signals carried by anygiven single circuit are not degraded by installation/maintenanceoperations carried out on other single circuits of the system. This isaccomplished by ensuring that each single circuit is housed and routedas a separate entity at a point in the network where re-entry forinstallation/maintenance purposes is possible. Thus, the presentinvention provides an optical fibre management system for managing aplurality of optical fibres, the system being such that the fibre(s) ofeach single circuit as hereinbefore defined is/are routed separatelyfrom the fibres of other circuits, whereby optical signals carried byany given single circuit are not degraded by handling operations carriedout on the fibres of other circuits.

In a preferred embodiment, each single circuit includes at least onesplice connecting first and second fibres, and wherein the splice(s) ofeach single circuit is/are housed in a respective splice tray.Advantageously, the or each first fibre leading to a given splice trayis housed in a respective fibre routing mechanism, and the or eachsecond fibre leading to said splice tray is housed in a respective fibrerouting mechanism. Different splice tray designs can be used. Thus, a"single circuit" splice tray is associated with a single circuit havingtwo first fibres, two second fibres and two splices; and a "singleelement" splice tray is associated with a single circuit having up toeight first fibres, eight second fibres and eight splices. In the firstof these cases, therefore, each splice tray houses two splices, wherebyeach of the fibre routing mechanisms associated with each splice trayhouses a respective pair of optical fibres.

Preferably, the splice trays are arranged in a stack, each splice trayhaving a main body portion for holding at least one splice and forstoring fibres leading to the or each splice, and a fibre entry/exitportion for feeding fibre to/from the main body portion. Conveniently,each splice tray is mounted in the stack so as to be movable from astacked position, in which it is aligned with the other trays, to firstand second operating positions in which the fibre entry/exit portion andthe main body portion respectively are accessible.

The invention also provides an optical fibre routing member formed witha plurality of input channels for housing respective input opticalfibres, and with a plurality of output channels for housing respectiveoutput optical fibres, wherein the input channels are positioned along afirst edge of the routing member, and the output channels are positionedalong a second edge of the routing member, and wherein the routingmember is formed with a curved guide for guiding the output fibres fromthe input channels to the output channels in such a manner that thefibres are not curved beyond minimum bend radius requirements for liveoptical fibre.

In a preferred embodiment, a common edge of the routing memberconstitutes the first and second edges.

Advantageously, the routing member further comprises holder means forhousing splitter means which splice the input fibres to the outputfibres, the curved guide means being positioned to guide the outputfibres from the splitter means to the output channels.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will now be described in greater detail, by way ofexample, with reference to the accompanying drawings, in which:

FIG. 1 is a perspective view, for one side, of an optical fibretelecommunications network node incorporating three splitter arraysub-assemblies each of which is constructed in accordance with theinvention;

FIG. 2 is a perspective view, form the opposite side, of the node ofFIG. 1;

FIG. 3 is a perspective view showing the node of FIGS. 1 and 2 mountedin a footway box in a storage position;

FIG. 4 is a perspective view similar to that of FIG. 3, but showing thenode 2 mounted in the footway box in its working position;

FIG. 5 is an exploded perspective view of one of the splitter arraysub-assemblies of the node of FIGS. 1 and 2;

FIG. 6 is a perspective similar to that of FIG. 5, but showing parts ofthe sub-assembly, then parts being in their operative positions;

FIG. 7 is a perspective view of one of the splice trays of the splitterarray sub-assembly of FIGS. 5 and 6;

FIG. 8 is a plan view showing the fibre entry/exit portion of the splicetray of FIG. 7.

FIG. 9 is a perspective view of one of the bend-limiting tube manifoldsof the splitter array sub-assembly of FIGS. 5 and 6;

FIG. 10 is a perspective view of one of the coupler array mats of thesplitter array sub-assembly of FIGS. 5 and 6;

FIG. 11 is a perspective view of the coupler array back cover of thesplitter array sub-assembly;

FIG. 12 is a plan view of the break-out tray which forms part of thenode of FIGS. 1 and 2;

FIG. 13 is an enlarged perspective view of part of the break-out tray;

FIG. 14 is a perspective view of another form of splice tray which canbe incorporated into a splitter array sub-assembly; and

FIG. 15 is a graph plotting error probability against system margin fora known type of splice tray.

DETAILED DESCRIPTION OF THE EXEMPLARY EMBODIMENTS

Referring to the drawings, FIGS. 1 and 2 show a node N forming part of aring topography PON. The node N includes a stack of three splitter arraysub-assemblies S₁, S₂ and S₃ and a break-out tray T. A 96 fibre cable C,which forms a ring (loop) centred on a local exchange (not shown),enters the break-out tray T via a cable entry portion 2 (see FIG. 12)after passing through a node base 1. The cable (then passes at leasttwice round a generally oval perimeter track 3 of the tray T, and leavesthe tray via the portion 2. The 96 fibres are housed in twelve flexibletubes (not shown) made of plastics material, each of the tubescontaining eight primary-coated fibres. As is described in detail belowwith reference to FIG. 12, the tray T includes a break-out region B inwhich individual fibre end portions, formed by cutting into one of thetubes, are led to the splitter array sub-assemblies S₁, S₂ and S₃. Inthis connection, it should be noted that the tray T stores a sufficientlength of the cable C so that, after cutting one of the tubes in themiddle of this stored length, and stripping back that tube to expose itsoptical fibres, each of the originally continuous fibres forms two fibreend portions whose length is sufficient to be led to the splitter arraysub-assemblies S₁, S₂ and S₃, and to leave spare fibre which can bestored for future use.

FIGS. 3 and 4 show the mounting of the node N in a footway box F, adome-shaped cover D being fixed to the node base 1 prior to mounting.

One of the splitter array sub-assemblies, S₁, is shown in detail inFIGS. 5 and 6. The other two sub-assemblies S₂ and S₃ are the same asthe sub-assembly S₁. The sub-assembly S₁ includes a stack of ten splicetrays 4, each of which is 8 mm thick. The trays 4 are supported (in amanner described below) by a stainless steel chassis 5 constituted by atop plate 5a, a base plate 5b and a back plate 5c. Each of the splicetrays 4 is a single circuit splice tray, that is to say, in use, it hastwo incoming optical fibres (one each for transmitting and receiving),and two outgoing optical fibres (one each for transmitting andreceiving). The three plates 5a, 5b and 5c are welded together, and thetop plate 5a of the sub-assembly S₁ can be fixed to the base plate 5b ofthe adjacent sub-assembly S₂ (not shown in FIG. 5 and 6) by means ofmounting bolts (not shown). Similar mounting bolts can be used to fixthe plate 5a of the sub-assembly S₁ and the plate 5b of the sub-assemblyS₃ to support means (not shown) in the node N.

The chassis 5 also supports an input splitter array mat 6, an outputsplitter array mat 7, and a splitter array back cover 8. In thisconnection, the input mat 6 carries (as is described below withreference to FIG. 10) fibres which carry telecommunications signals fromthe exchange to customers. These fibres are termed transmit fibres.Similarly, the output mat 7 carries fibres which carrytelecommunications signals from customers to the exchange. These fibresare termed receive fibres. The mats 6 and 7 are made of a flexiblepolymer, for example an elastomeric polymer such as injection mouldablezantoprene, or polyurethane. The back cover 8 is made of flexiblepolypropylene (which is also injection mouldable). This inherentflexibility ensures that, in use, the mats 6 and 7 are held firmlyagainst the chassis back plate 5c by the back cover 8.

As shown in FIG. 7, each splice tray 4 has a main body portion 9 and afibre entry portion 10 which also constitutes a clip-on test area. Fibreaccess to the main body portion 9 from the fibre entry portion 10 is viaa channel 11. The main body portion 9 is of oval configuration, havingan oval base 9a and an upstanding peripheral wall 9b. A hollow mandrel12 is provided on the base 9a adjacent to the entry channel 11. Themandrel 12 is of rounded square cross-section, is sized to ensureminimum bend requirements for live fibre passing around it, and has afibre inlet aperture 12a through which dark fibre can pass for internalstorage. A channel 13 is defined between the mandrel 12 and theperipheral wall 9b, the channel 13 leading to a further channel 14 whichleads around the inside of the wall to a splice holder region 15. Inuse, this region 15 houses a splice holder (not shown) for splicing twoincoming fibres to two outgoing fibres. A direction reversing channel 16leads from the channel 14 adjacent to the region 15 back to that portionof the channel 14 which adjoins the channel 13 adjacent to the mandrel12.

The fibre entry portion 10 of each splice tray 4 includes three fibreentry/exit ports 17a, 17b and 17c (see FIG. 8). Diverging channels 18aand 18b are provided to lead fibre between the port 17a and the channel11 via respective apertures 19a and 19b. These apertures 19a and 19bconstitute what are known as "clip-on apertures", and provide easyaccess to the associated fibres in order to measure the light passingthere along, and hence to determine the quality of the splices. Theseclip-on apertures, and associated light measurement apparatus, aredescribed in the specification of our International patent applicationWO 93/00600.

Similar diverging channels 20a and 20b are provided to lead fibrebetween the port 17c and the channel 11 via respective clip-on apertures21a and 21b. A single channel 22 is provided for leading fibre betweenthe port 17b and the channel 11. The channel 22 is not provided with aclip-on aperture.

Each splice tray 4 is also provided with a number of fibre retentiontabs 23 for holding fibre in the various channels 11, 13, 14, 16, 18a,18b, 20a, 20b and 22. One of these tabs (indicated by the referencenumeral 23a) is generally V-shaped, and extends from the curved end ofthe peripheral wall 9b remote from the mandrel 12 about halfway across,and above, that portion of the base 9a between that wall portion and themandrel.

Each tray 4 is pivotally mounted on the splitter array back cover 8 bymeans of a leash 24 and a retaining ring 25 which are moulded integrallywith the rest of the tray. The leash 24 of each tray 4 has two arms 24aand 24b joined together by a hinge 24c. Its retaining ring 25 is afriction fit within a groove 26 formed in the back cover 8 (see FIG.11). In use, a rod (not shown) passes through all the retaining rings 25and through apertures (not shown) in the top and base plates 5a and 5b.In this way all the splice trays 4 are retained by their back plates 5c,but each can be pivoted out away from the other trays in the stack toprovide access to its clip-on apertures 19a, 19b, 21a and 21b. In thisposition, the arms 24a and 24b take up a generally straightlineconfiguration (as opposed to the V-shaped configuration they have whenthe tray is in the stack). As the retaining ring 25 of a pivoted-outtray 4 is held in position by the retaining rod, the pivotal movement ofthe tray is limited by the leash 24 as its two arms 24a and 24bstraighten out. In the fully pivoted-out position, the fibre entryportion 10 of a tray 4 is exposed.

Each of the splitter array sub-assemblies S₁, S₂ and S₃ is associatedwith two fibres (four fibre end portions) of the eight fibres in the cuttube of the cable. The remaining two fibres (four fibre end portions)from the cut tube are stored in the break-out tray T as is describedbelow with reference to FIG. 13. As the cable C is in the form of aring, telecommunications signals can travel to/from the exchange ineither direction round the ring. For convenience, one Of the directionsis termed the main direction, and the other the standby direction. Inpractice, only main fibres will be used for normal signalling, thestandby fibres only being used in the eventuality of main fibre failure.

The two main fibre end portions associated with say the splitter arraysub-assembly S₁ pass from the break-out tray T to the lowest splice tray4 of that assembly, the fibre end portions being supported in, andprotected by, a bend limiting tube 27a (see FIG. 6). This bend limitingtube 27a is a proprietary item made of polypropylene ringed tubingwhich, though flexible, cannot easily be bent beyond minimum bend radiusrequirements for live fibre. The bend limiting tube 27a terminates inthe port 17a of the lowest splice tray 4, and its two fibre end portionsare led into the main body portion 9 via the channels 18a and 18b, theclip-on apertures 19a and 19b, and the channel 11. These fibre endportions are then spliced to the ends of a pair of fibres which (as isdescribed below) are associated with the mats 6 and 7. The two splicesare then positioned in a splice holder which is then mounted in theregion 15. The four fibres leading to the splices are then stored in themain body portion 9 of the tray 4 with two of the fibres (for examplethose from the break-out tray T) being led away from the splices in thechannel 14, and the other two fibres being led away from the splices viathe channel 13 and the reversing channel 16. A length of each of thefibres is stored in the main body portion 9 of the tray 4 by passingthese fibres one or more times round the mandrel 12 and under theV-shaped tab 23a. The fibres' natural resilience will ensure that theloops of fibre expand outwardly into a configuration of varying diameterturns. The provision of stored fibre permits a minimum of ten re-splicesof each of the splices to be carried out during the lifetime of theassembly.

The two fibres which are associated with the mats 6 and 7 leave the mainbody portion of the tray 4 via the channel 11. They are then led to theport 17c of the entry portion 10 via the clip-on windows 21a and 21b andthe channels 20a and 20b. These fibres are then led to the mats 6 and 7within a bend limiting tube 27c (see FIG. 6). One of these main inputfibres terminates on the input mat 6, where (as is described below withreference to FIG. 10) it is joined by splitter means to eight outputfibres. Similarly, the other of these main input fibres terminates onthe output mat 7, where it is joined by splitter means to eight outputfibres.

The two main fibre end portions and the associated pair of fibres towhich they are spliced constitute a single circuit, this circuitstarting in the break-out region B of the break-out tray T and finishingat the input to the splitter mats 6 and 7. Throughout the length of thissingle circuit, its fibres are routed separately from the fibres ofother circuits, so that single circuit management results. Thus, in thebreak-out region B of the break-out tray T the two main fibre endportions are separated (as described in greater detail below withrespect to FIG. 13) from the other cut fibre end portions. These twofibre end portions are then fed to the splitter array sub-assembly S₁within the bend limiting tube 27a, after which they are fed into thelowest splice tray 4 of that assembly. The two fibres spliced to thesemain fibre end portions are then fed to the splitter mats 6 and 7 withinthe bend limiting tube 27c. Clearly, the fibres carried .by this splicetray 4 form part of the single circuit. Consequently, the entire singlecircuit between the break-out point and the splitting point is separatedfrom all other circuits in the region where re-entry forinstallation/maintenance purposes is to be expected. This ensures thatoptical signals carried by this circuit are not degraded byinstallation/maintenance operations carried out on other circuits of thesystem.

The two standby fibre end portions associated with this splitter arraysub-assembly S₁ pass from the break out tray T to the second lowestsplice tray 4 of that assembly. Here, these two fibre end portions arespliced to two fibres which are led back to the mats 6 and 7 and so aretermed standby input fibres, and each of the standby input fibres isjoined by splitter means to the same eight output fibres as thecorresponding main input fibre. The fibre arrangement on this secondlowest splice tray 4 is the same as that for the lowest splice tray.Similarly, fibres enter and leave this splice tray 4 in bend limitingtubes 27a and 27c.

Thus, the two standby fibre end portions and the associated pair offibres to which they are spliced also constitute a single circuit, thiscircuit starting in the break-out region B of the break-out tray T andfinishing at the input splitter mats 6 and 7. As with the single circuitassociated with the main fibre end portions, this single circuit has itsfibres routed separately from other circuits throughout its length, sothat single circuit management results.

The remaining eight splice trays 4 in the sub-assembly S₁ of FIGS. 5 and6 are customer splice trays. As the fibre arrangement in each of thesecustomer splice trays 4 is the same, this will be described in detailfor only one of these trays. Thus, one of the output fibres from each ofthe mats 6 and 7 (that is to say a transmit fibre and a receive fibre)is led to the port 17c of a given customer splice tray 4 inside a bendlimiting tube 27c. These two fibres are led into the main body portion 9of the tray 4 via the channels 20a and 20b, the clip-on windows 21a and21b, and the channel 11. In use, these fibres are spliced to two fibresof a four-fibre blown fibre unit associated with a given customer. Sucha unit has four fibres in a single tube, the tube being fed between thecustomer and the node N by the well known fibre blowing technique (seeEP 108590). The customer's blown fibre unit is led to the port 17a ofthe splice tray 4 within a bend limiting tube 27a. The blown fibrecoatings are stripped away from the four fibres "downstream" of the port17a.

Two of the fibres within the unit (the two fibres which are to bespliced to the transmit and receive fibres from the mats 6 and 7, and soare termed live fibres) are fed to the main body portion 9 of the splicetray 4 via the channels 18a and 18b, the clip-on apertures 19a and 19b,and the channel 11. The two other fibres (which are spare fibres not forimmediate use) are fed to the main body portion 9 of the splice tray 4via the channels 22 and 11. All four fibres then pass round the mandrel12 within the channel 13, and then back to the mandrel after passingalong the channels 14 and 16. The two spare (dark) customer fibres passthrough the aperture 12a and are stored inside the mandrel 12. The twolive fibres pass round the mandrel 12, and are then spliced to thetransmit and receive fibres from the mats 6 and 7, the splices arestored in a splice holder, and the splice holder is positioned in theregion 15. As with the two lowest splice trays 4, each of the splicedfibres has a length to be stored (enabling up to ten re-splices to bemade during the lifetime of the assembly), these fibre lengths likewisebeing stored by looping them each one or more times round the mandrel 12and under the V-shaped tab 23a.

The two fibres leading to each of the customer splice trays 4 and thetwo fibres spliced thereto from the associated four-fibre blown fibreunit constitute a single circuit, this circuit starting at the output ofthe splitter mats 6 and 7, and terminating at the transmitter/receiverpair of the customer. Throughout the length of this single circuit, itsfibres are routed separately from the fibres of other circuits, so thatsingle circuit management results. Thus, from the exit of the splittermats 6 and 7, the output fibres from the splitter mats are fed to thesplitter array sub-assembly S₁ within the associated bend limiting tube27c. Within the associated splice tray 4, these fibres are spliced totwo fibres of the customer's four-fibre blown fibre unit. This tray 4thus houses only fibres forming part of a single circuit, and the tubeof the blown fibre unit separates the fibres of that circuit from fibresof other circuits of the system all the way to the customer's premises.Consequently, the entire single circuit between the output of thesplitter mats 6 and 7 and the transmitter/receiver pair of the customeris separated from all other circuits. In particular, in the region wherere-entry for installation/maintenance purposes is to be expected (thatis to say in the region of the node N), single circuit management isensured, so that optical signals carried by any given single circuit arenot degraded by installation/maintenance operations carried out on othercircuits.

In order to access the splices within a given splice tray 4, it isnecessary to remove the rod holding the retaining rings 25 in position,prior to the pulling that tray out of the stack sufficiently far to gainaccess to the splices. In this position, the tray 4 is maintained inposition by its bend limiting tubes.

The two spare customer fibres stored within the mandrel 12 of a givensplice tray 4 can be used to replace the two live fibres of thatcustomer in the event of these fibres failing. More importantly,however, they can be used to provide that customer with additional linesor service. (In this connection, it should be noted that each fibre paircan provide up to 32 lines using customer premises equipment (CPE)electronics such as an optical network unit (ONU) matched to an opticalline terminal (OLT) at the exchange. Each pair of fibres can alsoprovide a Megastream service.) In this case, the two spare fibres areremoved from their storage position within the mandrel 12, and are ledto the fibre entry portion 10 of the tray 4 via the channels 13 and 11.They then leave the tray 4, via the apertureless channel 22 and the port17b, and enter a bend limiting tube 27b (see FIG. 6). This tube 27b isrouted via the back cover plate 8 to another splice tray 4--usually asplice tray of another of the sub-assemblies S₂ or S₃ of the node N. Thetube 27b terminates at the port 17a of this tray 4, and the two fibresare led into the main body portion 9 via the channels 18a and 18b, theapertures 19a and 19b, and the channel 11. Here they are spliced to two"exchange" fibres, and all spare lengths of fibre are stored in the samemanner as that described above for the other splice trays. In thisconnection, the "exchange" fibres could be either a second pair offibres from the break-out tray T (direct exchange fibres), or a pair ofoutput fibres from the mats 6 and 7 (indirect exchange fibres).

These two spare fibres and the associated "exchange" fibres thusconstitute a single circuit, this circuit starting at the splice tray 4which originally stored the two spare fibres, and terminating either atthe break-out tray T or at the mats 6 and 7. In this case, the singlecircuit does not extend to the transmitter/receiver pair of thecustomer, as the two pairs of the four-fibre unit are housed in the sametube. The single circuit thus terminates at the point in the splice tray4 where the two pairs of fibres are separated, that is to say where amultiple circuit becomes two single circuits. Throughout the length ofthis single circuit, its fibres are routed separately from the fibres ofother circuits, so single circuit management results. Thus, from thepoint in the first tray 4 where the two pairs of fibres from thecustomer are separated, the "spare" fibres are routed separately fromthe two "live" fibres within the tray. On leaving that tray 4, the"spare" fibres pass to another splice tray 4 within an associated bendlimiting tube 27b. These fibres are spliced to "exchange" fibres in thissecond tray 4, and the two "exchange" fibres are passed either to thebreak-out tray T within a bend limiting tube 27a, or to the mats 6 and 7within a bend limiting tube 27c. In particular, in the region wherere-entry for installation/maintenance purposes to be expected (that isto say in the region of the node N), single circuit management isensured, so that optical signals carried by the single circuit are notdegraded by installation/maintenance operations carried out on othercircuits.

As each of the splice trays 4 is associated with a respective singlecircuit, these splice trays are termed single circuit splice trays.

The bend limiting tubes 27a, 27b and 27c of each of the splice trays 4are provided with respective support manifolds M (see FIGS. 6 and 9).Each manifold M is a sliding friction fit on a flanged portion (notshown) of the chassis back plate 5c, and is provided with an openaperture 28a for supporting the associated bend limiting tube 27a, andwith a pair of closed apertures 28b and 28c for supporting respectivelythe associated bend limiting tube 27b (if there is one) and theassociated bend limiting tube 27c. The manifolds M are made of injectionmoulded filled nylon.

FIG. 10 shows the input mat 6 of the sub-assembly S₁. The output mat 7of this sub-assembly, being of identical construction to the input mat6, will not be described in detail. The mat 6 includes an input slot 29for receiving the main input fibre, and an input slot 30 for receivingthe standby input fibre. These two slots 29 and 30 lead to an aperture31 which houses a 2×2 fused coupler (not shown). The two output fibresfrom this fused coupler are led via a curved channel 32 around a mandrel33. The mandrel 33 has a radius of 30 mm, and so fulfils the minimumbend requirements for live fibre. Each of the fused coupler outputfibres is spliced to an input fibre to a respective 1×4 planar coupler(splitter). The two splices are stored in a recess 35b.

The two planar couplers (not shown) are housed in an aperture 34adjacent to the aperture 31. The two fibres pass from the mandrel 33 totheir planar couplers via the curved end wall 35a of a recessed portion35 of the mat 6, and via respective curved slot 36. The eight outputfibres of the two planar couplers pass round the mandrel 33 via a slot37. These fibres then leave the mat 6 via respective output slots 38which fan out over the recessed portion 35 and the adjacent raisedportion which defines the curved end wall 35a.

The mat 6 thus forms a 2×8 splitter for the transmit fibres, with one ofits inputs being the main transmit input fibre and the other the standbytransmit input fibre. As mentioned above, only main fibres are used innormal operation, so the mat 6 acts as a 1×8 splitter. However, shouldthere be problems with the main fibre route, the mat 6 will still act asa 1×8 splitter with the standby receive fibre as its input fibre.

Similarly, the mat 7 constitutes a 2×8 splitter for the receive fibres.

FIG. 11 shows the splitter array back cover 8 of the sub-assembly S₁ ingreater detail. The back cover 8 is formed with a pair oflongitudinally-extending grooves 8a adjacent to that end remote from thegroove 26. These grooves 8a reduce the thickness of the back cover inthis end region, and so enhance the flexibility of the back cover,thereby ensuring that, in use, the back cover holds the mats 6 and 7firmly against the chassis back plate 5c. In this connection, it shouldbe noted that this end region of the back cover 8 is formed with anin-turned L-shaped flange 8b which can be snapped over grooves 28dformed in the manifolds M to hold the back cover to the chassis 5 withthe mats 6 and 7 firmly sandwiched therebetween.

The outer surface of the back cover 8 is also provided with a pluralityof longitudinally-extending ribs 8c, the base of each rib being formedwith a plurality of apertures 8d. These apertures 8d extend rightthrough the back cover 8 to its inside surface, and constitute a matrixof tie points for the attachment of cable ties which are used to tie thebend limiting tubes 27a, 27b and 27c to the sub-assembly S.

FIG. 12 shows the break-out tray T in greater detail. As mentionedabove, two loops of the cable are stored in the track 3, before thecable exits the break-out tray T via the entry portion 2, and one of thetubes of the cable is cut in the middle of its stored length. One of thecut fibres forms the main fibre for the splitter array sub-assemblyshown in FIGS. 5 and 6, and another the standby fibre for thatsub-assembly. The remaining fibres can be main and standby fibres forother splitter array sub-assemblies S₂ and S₃ of the node N, or can bestored around a mandrel 39 at that end of the tray T remote from thecable entry portion 2. The mandrel 39 has a rounded rectangularcross-section, and is sized so that fibre coiled therearound does notexceed minimum bend radius requirements.

The break-out region B of the tray T is formed with a plurality ofcurved upstanding fingers 40, adjacent pairs of which define sixteenfibre feed channels 41. The two fibre end portions that constitute themain fibres associated with the lowest splice tray 4 of the sub-assemblyS₁, are fed through the first channel 41 (that is to say through thechannel nearest the entry portion 2). Similarly, the two fibre endportions that constitute the standby fibres associated with the secondlowest splice tray 4 are fed through the second channel 41. (As thereare sixteen channels 41, the break-out tray T can handle sixteen pairsof fibre end portions, that is to say all the fibre end portions fromtwo cut tubes.) The two fibres then pass into the bend limiting tube 27aassociated with the lowest splice tray 4 of the sub-assembly S₁. Thistube 27a passes through an aperture 42 in a raised portion 43 of thebreak-out region B (see FIG. 13), and is tied in place by ties (notshown) associated with a further aperture 44.

A preferred form of TPON includes a 32-way split, that is to say eachfibre from the exchange serves 32 actual customers via one or moresplitting (flexibility) points such as the node N described above. Asthe node N defines an 8-way split, it could be used as a primarysplitting point, in which case each of the "customer" fibres leaving thenode would lead to a respective secondary splitting point. In this case,each of the single circuits associated with a customer tray 4 wouldstart at the output of the splitter mats 6 and 7, and terminate at theinput to the secondary splitting point. Alternatively, if the "customer"fibres leading to the secondary splitting point are fed back into amulti-fibre tube, the single circuit terminates at the entry to thistube. In either case, in the region where re-entry forinstallation/maintenance purpose is to be expected, single circuitmanagement is ensured.

Each of the secondary splitting points would be similar to the node N,but each incoming fibre would be split four ways rather than eight ways.As the outgoing fibres from a primary node do not go directly tocustomers, the terms "customer splice trays" and "customer fibres" usedabove should be taken to mean splice trays and fibres associated witheither actual customers or with downstream splitting points. Of course,in the preferred 32-way split form of TPON, the nodes N could also besecondary nodes. In this case, there would be four nodes N, each servingeight actual customers, and the four secondary nodes would be served viaa 4-way split primary node. Here again, the primary node, would besimilar to the node N, but each incoming (exchange) fibre would be splitfour ways rather than eight ways.

It will be apparent that the type of splitter array sub-assemblydescribed above is extremely flexible in that it can readily be adaptedto suit different requirements. In particular, it is adaptable to anysplitting ratio by varying the number of splice trays used and the sizeand form of the splitter array mats 6 and 7. Moreover, by co-locatingseveral splitter array sub-assemblies in a node, splitting from aplurality of exchange fibres can be accomplished at any given point,using different splitting ratios in each sub-assembly if required.

It will be apparent that the arrangement described above ensures singlecircuit management. In particular, in the region of a node N (that is tosay where re-entry for installation/maintenance purposes is to beexpected) single circuit management is ensured, so that optical signalscarried by any single circuit are not degraded byinstallation/maintenance operations carried out on any other circuit.

An important advantage of the sub-assemblies described above, is thatthe splitters and the associated fibres can all be factory fitted. Thus,the fused and planar couplers and their associated fibres can be madeand positioned in the mats 6 and 7, and the associated fibres can be ledto their splice trays 4 within bend limiting tubes--all at the factory.When the sub-assembly is to commissioned, the fitter needs only to cutone or more tubes of the cable C, feed main and standby fibre endportions to the lowest two splice trays 4 of the sub-assembly, storespare cut fibre end portions in the break-out tray T, splice the mainand standby fibre end portions to the main and standby input fibresalready present in the two splice trays, and to splice "customer" fibresto the fibres already present in the other splice trays 4. In this way,the amount of skilled work which has to be carried out on site isreduced to a minimum. In particular, the fitter does not need to carryout any intricate splicing for splitting purposes.

The sub-assembly described above could also be adapted for use in a spurjoint. In such a case, no splitting is required, so the sub-assemblywould not include the mats 6 and 7. In a first type of spur joint, alltwelve tubes of the fibre cable C would be cut, thereby forming twelvemain fibre tube ends and twelve standby fibre tube ends. The fibres ofsix of the main fibre tube ends would then be spliced to the fibres ofsix of the standby fibre tube ends in single element splice trays 45 (asis described below with reference to FIG. 14). The fibres of theremaining six main fibre tube ends are then spliced to "customer" fibresin 24 single circuit splice trays 4. Similarly, the fibres of theremaining six standby fibre tube ends are spliced to 48 "customer"fibres in 24 single circuit splice trays 4. Thus, two fibres are fed, inbend limiting tubes, from a breakout tray (not shown) to each of the 48single circuit splice trays 4, where they are spliced to "customer"fibres in a similar manner to that described above with reference toFIGS. 5 and 6. As with the customer splice trays 4 discussed above,single circuit management results for each of these 48 splice trays.

A respective main fibre tube end and a respective standby fibre tube endare fed from the break-out tray to each of the single element splicetrays 45 (see FIG. 14), each tube end being in a respective bendlimiting tube (not shown, but similar to the bend limiting tubes 27a,27b and 27c). Each tray 45 has a main body portion 46 and a tube entryportion 47. The main body portion 46 is of oval configuration, having anoval base 46a and an upstanding peripheral wall 46b. Fibre access to themain body portion 46 from the tube entry portion 47 is via a channel 48.Channels 49, 50, 51 and 52 are provided in the main body portion 46 toguide both main and standby fibres to a pair of splice holder regions53. The channel 51 is a direction reversing channel, and permits mainand standby fibres to approach each of the splice holder regions 53 fromopposite directions.

Each single element splice tray 45 is also provided with a number offibre retention tabs 54 for holding fibre in the various channels 49 to52.

The tube entry portion 47 of each single element splice tray 45 includestwo tube entry/exit ports 55a and 55b. Channels 56a and 56b are providedto lead fibre between the ports 55a and 55b and the channel 48.

The single element splice tray 45 is provided with a leash 57 and aretaining ring 58 (similar to the leash 24 and the retaining ring 25 ofthe tray 4). The leash 57 permits the tray 45 to be pivoted out of astack of trays to enable access to the tube entry portion 47.

In use, a main fibre tube end is led to the port 55a of each of thesplice trays 45, and a standby fibre tube end is led to the port 55b ofeach of the splice trays 45. Inside each tube entry portion 47, thetubes are cut away to expose the fibres. The fibres are then fed intothe main body portions 46 of the trays, where they are spliced. Theeight splices in each tray 45 are then positioned, four in each of apair of splice holders, and the splice holders are then mounted in theregions 53. The fibres leading to the splices are then stored in themain body portions 46 of the trays 45. A length of each of the fibres isstored in the main body portion 46 of the associated tray 45 by passingthese fibres one or more times round an upstanding mandrel 59 and underthe tabs 54. The fibres' natural resilience will ensure that the loopsof fibre expand outwardly to a configuration of varying diameter turns.The provision of stored fibre permits re-splicing to be carried outduring the lifetime of the assembly.

In a modified version of the spur joint described above, only six of thetubes are cut, the fibres in these tubes being spliced to "customer"fibres in 48 single circuit splice trays 4 as described above. Theremaining six uncut tubes are looped around a break-out tray.Alternatively, instead of using 48 single circuit splice trays 4, sixsingle element splice trays 45 could be used. This alternative would,however, only be used in cases where there is no need to access spurjoints for future use.

Obviously, for either type of spur joint, the number of fibres formingthe spur can be varied. For example, the spur could be formed from thefibres of one cut tube. In this case the spur would contain 16 fibres(eight main fibres and eight standby fibres from a single cut tube) and88 fibres (from the remaining eleven tubes--either cut and spliced oruncut and coiled) would continue through on the ring. In each case,however, single circuit management is ensured for each of the singleelement splice trays 45. Thus, the single circuit associated with agiven single element splice tray 45 starts and terminates at thebreak-out tray T, a respective main fibre tube end being fed to thesplice tray 45 in a bend limiting tube, where its fibres are spliced tothe fibres of the standby fibre tube and which, in turn, is fed back tothe break-out tray within a bend limiting tube. The single circuit hereis, therefore, constituted by a plurality of fibres (eight in theexample above).

We claim:
 1. An optical fibre management system for providing singlecircuit management maintenance services to each single circuit in asystem of plural optical signal distribution circuits withoutsubstantial disturbance of any other circuit, said system comprising:aplurality of fibre service trays, each tray containing optical fibre,the optical fibre of each tray being included in only a single opticalsignal distribution circuit.
 2. A system as in claim 1 wherein eachsingle circuit includes at least one splice connecting first and secondfibres, and wherein the at least one splice of each single circuit ishoused in a respective splice tray.
 3. A system as in claim 2 whereinplural splice trays are arranged for radial movement into and out of astack, each splice tray having a main body portion for holding at leastone splice and for storing fibres leading to each such splice, and afibre entry/exit portion for feeding fibre to/from the main bodyportion.
 4. A system as in claim 3 wherein each splice tray is mountedin a stack so as to be movable from a stacked position, in which it isaligned with the other trays, to first and second operating positions inwhich the fibre entry/exit portion and the main body portionrespectively are accessible.
 5. A system as in claim 2 wherein eachfirst fibre leading to a given splice tray is housed in a respectivefibre routing mechanism, and each second fibre leading to said givensplice tray is housed in a respective fibre routing mechanism.
 6. Asystem as in claim 5 wherein each splice tray houses two splices,whereby each of the fibre routing mechanisms associated with each splicetray houses a respective pair of optical fibres.
 7. A method forproviding single circuit management maintenance services to each singlecircuit in a system of plural optical signal distribution circuitswithout substantial disturbance of any other circuit, said methodcomprising:separately routing the optical fibre of each single circuitthrough its own dedicated fibre access service tray which does notcontain fibre from any other circuit; and servicing optical fibre in agiven single circuit without substantial disturbance to optical fibre inany other circuit by individually accessing the service tray dedicatedto such given single circuit.